Better defaults with raw plot

[danieltomasz]

The PR #403 seems to fix the problem with canvas and raw_plot,
but since it is using under hood mne.viz.set_browser_backend('qt'), it would be great to have similar defaults as mne.viz.plot_raw(raw),

currently mnelab` is trying to show all channels with some non-default scaling, so with long HD-EEG it make it very hard to see what showing with eeg

Results from _ = mne.viz.plot_raw(raw)

Below mnelab
by default it is using data.plot(events=events, n_channels=258)

[cbrnr]

Should we use a default of 20 channels as well? Maybe it would be nice to customize the default settings (like number of channels and time segment per page) as mentioned in #175?